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More recent editions of his system are in copyright, and the name "Dewey," "Dewey Decimal," "Dewey Decimal Classification" and "DDC" are registered trademarked by OCLC, who publish periodic revisions.

LibraryThing's MDS system is based on the classification work of libraries around the world, whose assignments are not copyrightable. MDS "scheduldes" (the words that describe the numbers) are user-added, and based on public domain editions of the system.

The Melvil Decimal System is NOT the Dewey Decimal System of today. Wordings, which are entered by members, can only come from public domain sources. The base system is the Free Decimal System, a public domain classification created by John Mark Ockerbloom. Where useful or necessary, wording comes from the 1922 edition of the Dewey Decimal System. Language and concepts may be changed to fit modern tastes, or to better describe books cataloged. Wordings may not come from in-copyright sources.